1. Field of the Invention
The present invention related to a quantizer for use in image coding. It is known to scale discrete cosine transformation coefficients in video coding applications to conserve bandwidth.
2. Introduction
Known systems either scale by a small constant, such as divide by 8, or scale by a linear scaling factor that is twice a quantization parameter (2×Qp). Scaling by the small constant does not achieve significant bandwidth savings. Scaling by the 2*Qp linear scaling function achieves significant bit savings, but results in poor image quality at lower and mid-level Qp values particularly in the chrominance video signals. Accordingly, there is a need in the art for a quantizer characterized by a scaling function that achieves good signal quality, and achieves bit rate savings for all values of Qp particularly for chrominance.
An encoder and decoder must use the same quantization parameter to encode and decode video information intelligibly. Known systems report changes to the quantization parameter with codes that cause changes in Qp with a uniform step size, regardless of the value of Qp. Experience teaches that, at low values of Qp, changes in Qp are relatively small. However, for large values of Qp changes in Qp values are relatively large. Systems that allocate additional bits to report the larger Qp changes waste bandwidth at the lower Qp values where the large changes do not occur. However, systems that limit the number of bits available to coding Qp changes may become saturated if larger changes must be coded. Accordingly, there is a need in the art for a quantizer that reports both large and small changes in Qp with a minimum number of bits.